1. Field of the Invention
The present invention relates to a light wavelength shifter device and, more particularly, to an optical waveguide type light wavelength shifter device of a so-called guided mode matching type.
2. Description of Background Information
Second harmonic generator (abbreviated to SHG) is widely used as a light wavelength shifter device, which has an optical waveguide composed of a nonlinear optical crystal so that a light wave is propagated into a minute region for the generation of second harmonics with high efficiency. As this type of light wavelength shifter devices, two different types are known according to methods of phase matching: one employs a guided mode matching method in which a nonlinear polarized wave derived from a primary incident light is equalized in phase velocity with a second harmonic wave so that phase matching is made between a fundamental wave, i.e., a guided mode wave of the primary incident light and guided mode wave of the second harmonic wave; and the other is a type using a Cerenkov radiation method in which phase matching is made between a guided mode wave of the fundamental wave and a radiation mode wave of the second harmonic wave.
FIG. 1 illustrates, for example, an SHG of a ridge waveguide type, which comprises a core 1 made of lithium niobate (LiNbO.sub.3), a nonlinear optical crystal, and a cladding 2 of lithium tantalate (LiTaO.sub.3), an optical crystal material. In operation, a fundamental wave of a primary light is incident on one end of a ridge 3 or rib projection of the core 1. Then, a resultant second harmonic wave and a remaining fundamental wave component emerge from the other end of the ridge 3.
FIG. 2 is a cross-sectional view of the ridge type SHG. The core 1 of lithium niobate having a refractive index n.sub.1 is mounted on the cladding 2 of lithium tantalate which has a refractive index n.sub.2. In the guided mode matching, the matching wavelength .lambda.m of the primary incident light for shifting the primary incident light to a second harmonic wave having a wavelength of a half the primary incident light is expressed as a function of the overall height H of the core 1 and the refractive indices n.sub.1 and n.sub.2 of the core 1 and cladding 2.
More specifically, the matching wavelength .lambda.m of the primary light is determined by parameters n.sub.1, n.sub.2, d, H, and h as shown in FIG. 2. As the dimensions d, H, and h of each device are determined during the production, the matching wavelength .lambda.m will be determined uniquely if the two refractive indices n.sub.1 and n.sub.2 are constant.
As a light source of the fundamental wave, it is practical to use a semiconductor laser diode. However, the output wavelength of the semiconductor laser is usually not constant and, for instance, has a variation about 10 nm (nanometers). The wavelength is also affected by the factors of current and temperature. Hence, in the case of an SHG in which the matching wavelength .lambda.m of the primary light is uniquely determined, it will be difficult to attain phase matching, and the resultant second harmonic wave will become unstable.